RF communications systems are well known in the art. In many such systems, as in mobile applications, vehicles are equipped with mobile transceivers that allow their operators to communicate with one or more base stations or other similarly equipped vehicles in the system. These vehicles are often equipped with an automatic vehicle location system. One such vehicle location system is Long Range Navigation (LORAN-C), which is a low frequency (100 KHz) system originally intended to provide navigation capability to marine craft operating in the coastal waters of the continental United States and on the great lakes.
The LORAN-C system utilizes a group of stations transmitting synchronized pulse signals at a common repetition rate. These signals are received and time stamped by the vehicle's LORAN-C receiver. Utilizing the time difference between the receipt of these signals the receiver determines the location of the vehicles in which it is installed. This location information is subsequently routed to the mobile transceiver for transmission to a dispatch center, where location information of all vehicles in a fleet are processed.
Since the number of LORAN-C transmitters are limited and are sparsely located in the AVL system, the incoming signals at the vehicle's antenna are of very low levels (typically an average signal to noise ration of -15 to -10 dB) and must be effectively coupled to the receiver's input port. Furthermore these antennas must be more accurately designed and installed in order to meet the overall system requirements. A quarter wavelength antenna at 100 KHz is 750 meters in length. Such an antenna is physically impossible to install on a vehicle. Therefore an "E-FIELD PROBE" of a typical length of 0.5 meters is used. The effective impedance of this antenna is approximately 250 K.OMEGA.. Thus an active coupler is required to provide; impedance matching to a 50.OMEGA. coax cable, amplification of the received signal, and pre-selectivity of the received signal.
The objective of AVL systems is multifold. One is to determine the general location of a vehicle in order to identify the closest repeater to be used in communications with that vehicle. Another is, to closely monitor the routes travelled by vehicles in order to verify adherence to schedules. This monitoring of travel routes is not well received by the operators of vehicles in an AVL system and a number of different approaches have been employed by operators of such vehicles to falsify the location information sent in by the mobile radio. The easiest and most detrimental deactivation is antenna sabotage. The usual method of sabotage is to loosen the antenna coupler. This allows the AVL system to respond with ID and data other than valid vehicle location rendering the system useless.
A need therefore exists for a means to evaluate the coupling status of antennas in communication systems such as AVL systems and report this status to a base so that remedial action may be taken. Such evaluation and indication of the coupling status of antennas would greatly enhance communication systems' throughout and effectiveness. This invention provides such means and along with it other features valuable to users of such systems.